The H(2 resonances of the two active site histidines have been observed to disappear upon addition of uridine vanadate and to be replaced by four others. These were assigned as the unprotonated and protonated forms of the two histidines in slow exchange. However, these resonances did not titrate in the pH range of 5.0-8.0 - a fact that the authors could not adequately explain. We suspect that the two peaks represent two conformations of either the enzyme or inhibitor rather than differences in protonation states and we propose to use 15N labeled protein to observe the protonation states of these four peaks.